1. Field of the Invention
The present invention relates to an optical material which comprises an N-alkyl-substituted maleimide-olefin copolymer, and which is superior in transparency, heat resistance, surface hardness, mechanical strength, and other properties.
2. Description of the Related Art
Heretofore, optical materials are generally made of glass. Recently, transparent polymer materials have come to be used for optical materials in view of the productivity, light-weight, cost and so forth.
Such polymer materials includes polymethyl methacrylate (hereinafter referred to as "PMMA") and polycarbonate (hereinafter referred to as "PC").
PMMA, however, is limited in its use because of its insufficient heat-resistance resulting from its glass transition temperature (Tg) of about 100.degree. C., although it has superior optical characteristics.
PC, which has a Tg of about 140.degree. C. and has relatively high heat resistance, involves the disadvantages of poorer optical characteristics in comparison with PMMA, low surface hardness causing susceptibility to scratching, low weatherability, low moldability, and so forth.
On the other hand, maleimide type copolymers are being studied comprehensively because of its high heat resistance. For example, copolymerization of the aforementioned methyl methacrylate with N-aromatic-substituted maleimide is disclosed in Japanese Patent Publication No. Sho 43-9753, Japanese Patent Application Laid-Open Nos. Sho 61-141715, Sho 61-171708, and Sho 62-109811; and copolymerization of styrene resins with N-aromatic-substituted maleimide is disclosed in Japanese Patent Application Laid-Open Nos. Sho 47-6891, Sho 61-76512, and Sho 61-276807. The resins produced by these methods are improved more in heat resistance with the higher content of N-aromatic-substituted maleimide, but thereby causing problems of brittleness, low moldability, lowered transparency, and so forth, thus being limited in use for optical materials.
After comprehensive study regarding the above problems, it was found that an optical material comprising an N-alkyl-substituted maleimide-olefin type copolymer solves the problems, and the present invention has been accomplished.